1. Field of the Invention
The present invention relates to a color balancing circuit for use in a flat color display panel such as an electroluminescence (EL) panel employing organic EL elements.
2. Description of the Related Art
Organic EL panels are also known as organic light-emitting diode (OLED) panels. Other known types of flat panel displays include liquid crystal display (LCD) panels, non-organic light-emitting diode (LED) panels, and plasma display panels (PDPs). Two desirable qualities of these displays are an adjustable overall brightness and an adjustable balance of the brightness of the three primary colors red, green, and blue, so that colors can be accurately reproduced.
Japanese Patent Application Publication No. 2001-42823 discusses the driving of an organic EL multicolor display panel having a passive matrix structure. The panel has red, green, and blue electroluminescent elements (EL elements) disposed at the intersections of a plurality of column electrodes (also referred to as anode lines or drive lines) with a plurality of row electrodes (also referred to as cathode lines or scanning lines). When a direct-current (DC) driving voltage exceeding the emission threshold of an EL element is applied to the EL element, it emits light with a brightness proportional to the current that flows from the column line to the row line in response to the applied voltage. If the applied voltage is less than the emission threshold voltage, no current flows and the emission brightness remains zero.
The red, green, and blue EL elements are arranged so that all the EL elements in the same column are of the same color. A first potential and a higher second potential are selectively supplied to the row electrodes. A third potential that provides an offset voltage below the emission threshold voltage of the EL elements and a current source that provides driving current are selectively connected to the column electrodes. The driving current and the third potential are variable and can be adjusted to different values for the red, blue, and green columns, to equalize the voltage changes in the different columns, thereby improving the light emission rise characteristic. This arrangement permits adjustment of both overall brightness and color balance, but separate adjustment circuitry is required for each of the three primary colors.
Japanese Patent Application Publication No. 2001-134255 discusses the automatic adjustment of brightness according to the user's needs and ambient conditions (surrounding brightness) in a flat display panel such as an LCD panel with an adjustable backlight. The flat display panel has a display screen and a sensor disposed near the display screen for sensing surrounding brightness. An automatic (primary) adjustment of the brightness of the display screen is performed according to a signal output from the sensor. The flat display panel also has means by which the user can set the brightness of the display screen, means for setting the brightness characteristic of the display screen according to the ambient brightness detected by the sensor and the brightness value set by the user, and means for automatically adjusting the brightness of the display screen, after it has been set by the user, according to the brightness characteristic and the signal output from the sensor. This scheme provides a convenient brightness adjustment, but color balance must be adjusted by completely separate means.
Japanese Patent Application Publication No. 07-129100 discusses a simplified adjustment of the brightness of a color LED lamp panel. The lamp panel module has a plurality of picture elements (pixels), each comprising red, green, and blue LEDs that can combine to display an arbitrary color. The lamp panel module includes respective dimmer circuits in the red, green, and blue LED control circuits for independently controlling and adjusting the brightness of the red, green, and blue light, and has means for frequency control of the dimmer circuits. Color balance can thereby be adjusted, but this system fails to provide a single convenient adjustment for overall image brightness.
Japanese Patent Application Publication No. 08-286636 discusses the adjustment of the brightness of a plasma display panel. The mechanism by which a plasma display panel emits light is electrical discharge in a gas. Different pixel intensities are obtained by controlling the number of discharges on a pixel-by-pixel basis according to pixel data. Brightness is adjusted by doubling, tripling, or quadrupling the number of discharges, and also by multiplying the pixel data by a continuously variable gain factor. This scheme provides a continuous brightness adjustment over a wide range, but does not provide for adjustment of color balance.
In an organic EL panel, color balance and overall brightness can both be adjusted by adjusting the driving current supplied to the red, green, and blue light-emitting elements. If a separate adjustment is made for each primary color, an arbitrary desired color balance can be obtained, but separate adjustment circuitry is required for each color and overall brightness adjustment is inconvenient, because three separate adjustments are necessary. If a single adjustment of the driving current for all three primary colors is made, however, then although brightness adjustment is convenient and the amount of adjustment circuitry can be reduced, the color balance cannot be adjusted to suit the user's preferences, or to compensate for fabrication variations or aging changes.
For use in a flat color display panel such as an organic EL panel, it would be desirable to have a color balancing circuit capable both of adjusting the red, green, and blue driving currents individually to obtain a desired color balance, and of adjusting all three of the driving currents together to obtain a desired display brightness.